1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor that is used to form an image according to an electrophotography process and image forming apparatus providing the same
2. Description of the Related Art
In electrophotographic image forming apparatus used frequently as copy machines and printers, a surface of an electrophotographic photoreceptor (hereinafter simply referred to as photoreceptor as well) is charged uniformly by a charging unit, the surface is exposed to form an electrostatic latent image corresponding to image information, the electrostatic latent image is developed with fine particles called toner to form a toner image that is a visible image, a formed toner image is transferred on a toner receiving material such as transfer paper and fixed, and thereby an image is formed on the toner receiving material.
An electrophotographic photoreceptor is formed by laminating a photosensitive layer including a photoconductive material on an electrically conductive substrate. As a photoconductive material of the electrophotographic photoreceptor, so far, inorganic photoconductive materials such as selenium (Se), cadmium sulfide (CdS) and zinc oxide (ZnO2) have been used. However, since there are problems of toxicity and so on in these inorganic photoconductive materials, in recent years, electrophotographic photoreceptors that use an organic photoconductive material (hereinafter in some cases referred to as organic photoreceptor) that is nontoxic, excellent in the film forming property and wide in a material selection range have been actively developed.
As the organic photoreceptor, a functionally-separated photoreceptor in which a charge generation function and a charge transport function, respectively, are carried out by different substances is used. The functionally-separated photoreceptor is roughly divided into a single layer photoreceptor that has a photosensitive layer in which a charge generating substance that has the charge generation function and a charge transporting substance that has the charge transport function are dispersed together in a resin that is called a binder resin and has the binding property; and a laminate type photoreceptor that has a photosensitive layer in which a charge generation layer in which a charge generating substance is dispersed and a charge transport layer in which a charge transporting substance is dispersed are laminated.
The laminate type photoreceptor has advantages in that a photosensitive layer can be easily designed and an electrophotographic photoreceptor excellent in the sensitivity and stability can be relatively readily prepared; accordingly, the laminate type photoreceptor forms a main stream of the organic photoreceptors. Furthermore, the single layer photoreceptor, since the photosensitive layer is formed of a single layer, in comparison with the laminate type photoreceptor, is higher in the productivity, can be manufactured at lower manufacturing costs, and, since a positive charge process in which at charging ozone that is a hazardous material is generated with difficulty can be used, is being put into practical use.
As performance required for an electrophotographic photoreceptor in an image forming process, the superiority in the electrical characteristics such as the charging property, the sensitivity and the responsiveness can be cited. Furthermore, in the image forming apparatus, since the image forming process is repeated many times, the photoreceptor is demanded to be stable in the electrical characteristics and excellent in the electrical durability even in repetition use.
However, existing photoreceptors have disadvantages in that the electrical durability is insufficient and when the photoreceptors are repeatedly used, the fatigue and deterioration such as a decrease in the charging potential, an increase in the rest potential and deterioration of the sensitivity are caused to induce deterioration of image quality, resulting in being incapable of using over a long term.
Regarding reasons for the fatigue and deterioration of the photoreceptor, several factors can be cited. Among these, an influential factor is in that oxidizing gases such as ozone, nitrogen oxides, chlorine oxides, and sulfur oxides that are emitted from a corona discharge type charging device (herein after referred to as corona discharger) that is used as charging means when a photoreceptor is charged in the image forming process greatly damage a photosensitive layer. It is considered that the oxidizing gases react with a charge transport material to generate ion pairs that accompany electron transfer or are absorbed by the charge generation material to induce a decrease in the charging potential, an increase in the rest potential, deterioration of the sensitivity, and deterioration of the resolution power owing to a decrease in the surface resistance. As a result, the image quality is remarkably deteriorated and the lifetime of the photoreceptor is shortened.
In order to overcome the problem of the fatigue and deterioration of the photoreceptor due to the oxidizing gases, an idea is proposed in that the oxidizing gases generated from a corona discharger is efficiently evacuated and replaced by an inert gas to a photosensitive layer, and thereby an adverse affect of the oxidizing gases on the photoreceptor is suppressed. However, in order to evacuate the oxidizing gases, an evacuation unit has to be newly disposed in the image forming apparatus. However, a problem causes that a process as a whole and a system becomes complicated.
Furthermore, in order to suppress the fatigue and deterioration of the photoreceptor from occurring owing to the oxidizing gases, a particular compound is added to a photosensitive layer. It is proposed to add, for instance, an anti-oxidant such as a hindered phenol compound (Japanese Unexamined Patent Publication JP-A 62-105151 (1987)) and an aromatic amine compound such as N,N′-diphenyl-p-phenylene diamine (Japanese Unexamined Patent Publication JP-A 63-216055 (1988)); and a light stabilizer such as a hindered amine compound (Japanese Unexamined Patent Publication JP-A 63-18355 (1988)) and a benzotriazole compound to a photosensitive layer. Here, the hindered phenol compound is a phenol compound that has a bulky substituent group such as a side chain-like alkyl group, cyclo-alkyl group, aryl group or heterocyclic group in an ortho-position of a phenolic hydroxy group. Furthermore, the hindered amine compound is an amine compound in which a hydrogen atom of an amino group is substituted with a bulky substituent group such as a side chain-like alkyl group, cyclo-alkyl group, aryl group or heterocyclic group.
Furthermore, in different related art, it is proposed to add a trialkyl amine compound (Japanese Unexamined Patent Publication JP-A 63-4238 (1988)), a t-amine compound having a particular structure such as 2-(N,N-dibenzylamino) ethanol (Japanese Unexamined Patent Publication JP-A 03-172852 (1991)) and the like to a photosensitive layer.
The photoreceptors disclosed in the JP-A Nos. 62-105151, 63-216055, 63-18355, 63-4238 and 03-172852 have problems as shown below. Even though compounds disclosed in JP-A Nos. 62-105151, 63-216055, 63-18355, 63-4238 and 03-172852 are used, the fatigue and deterioration of the photoreceptor due to the oxidizing gases such as ozone and nitrogen oxides cannot be sufficiently suppressed from occurring; that is, after the repeated use thereof, the decrease of the charging potential and the increase of the rest potential are generated. Furthermore, when the compounds disclosed in JP-A Nos. 62-105151, 63-216055, 63-18355, 63-4238 and 03-172852 are added to a photosensitive layer, the deterioration of the sensitivity and the deterioration of the electrical characteristics such as the increase of the rest potential owing to the deterioration of the responsiveness are caused; accordingly, there is a problem that from the beginning of use, practically sufficient sensitivity and responsiveness cannot be obtained.